[llvm] [ARM/X86] Standardize the isEligibleForTailCallOptimization prototypes (PR #90688)

[Eli Friedman via llvm-commits]

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@@ -2723,11 +2722,19 @@ bool MatchingStackOffset(SDValue Arg, unsigned Offset, ISD::ArgFlagsTy Flags,
 
 /// Check whether the call is eligible for tail call optimization. Targets
 /// that want to do tail call optimization should implement this function.
+/// Note that musttail calls are not checked for eligibility, so the backend
+/// must support forwarding arguments of any type.
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efriedma-quic wrote:

This doesn't really get the relevant point across... I would say:

This function is also used to reject musttail calls the backend does not know how to emit.  musttail calls should be supported for all combinations of argument/return type.

https://github.com/llvm/llvm-project/pull/90688